Legal Question in Criminal Law in California

Unless someone in your home has a probation or parole condition allowing warrantless searches, don't open it at all. Make them kick it down.

If they have a warrant or other legal grounds to enter, they must comply with the "knock notice" requirements. They must knock or otherwise announce their presence, wait a reasonable period of time and then in refused entry (either by verbal or other actions that indicate non-compliance or after the passage of a reasonable period of time), they can force entry.

If they do not have a warrant or other legal grounds to enter, you do not have to allow entry. If they force the door open, the time to fight is in court, not on your front porch. Be safe.